The field of the invention pertains to the control of the rotational velocity of relatively slow speed brushless alternating current electric servo motors. In particular, the invention is directed to eliminating rotational acceleration and deceleration within each rotation as much as possible. The application to which the preferred embodiment described below is directed is one step in the manufacture of circuits on semiconductor wafers, however, the invention is not limited to this application.
One method of etching circuits onto semiconductor wafers incorporates a step during which the wafer is rotated at a constant velocity to distribute a viscous fluid evenly over the surface of the wafer. The quality of the etching process is directly affected by variations in the thickness of the fluid film. The thickness variations are influenced by several parameters including the instantaneous velocity variations in the rotation of the wafer. Typical rotational speeds for this processing step are in the range of 30 to 60 rpm. A subsequent step in processing the wafer requires that the wafer spin at a high rate, in the range of 5,000 to 10,000 rpm.
One method for rotating the wafer utilizes a motorized spindle driven by an electric motor. Of the different types of electric motors available, a brushless alternating current servo motor is preferred. Electric motors with brushes emit carbon and copper particles as well as ozone as a byproduct of their inherent commutation operation. These emissions contaminate the wafer and degrade the semiconductor manufacturing process, rendering brushed motors unsuitable. Of the range of brushless motors, brushless alternating current (AC) servo motors offer a unique combination of small size, high efficiency and low torque ripple.
The choice of a brushless AC motor alone is not sufficient to minimize instantaneous velocity variations to the extent required for accurate semiconductor wafer processing. Previous configurations employing a brushless AC motor and conventional quadrature digital encoder have not proven adequate to provide the degree of velocity consistency required for the process step. Attempts to increase the resolution of conventional encoders have caused bandwidth limitations which render the encoders inoperable at high speed.